It's never used.

When `catch_fatal_errors` catches a `FatalErrorMarker`, it returns an
`ErrorGuaranteed` that is conjured out of thin air with
`unchecked_claim_error_was_emitted`. But that `ErrorGuaranteed` is never
used.

This commit changes it to instead conjure a `FatalError` out of thin
air. (A non-deprecated action!) This makes more sense because
`FatalError` and `FatalErrorMarker` are a natural pairing -- a
`FatalErrorMarker` is created by calling `FatalError::raise`, so this is
effectively getting back the original `FatalError`.

This requires a tiny change in `catch_with_exit_code`. The old result of
the `catch_fatal_errors` call there was
`Result<Result<(), ErrorGuaranteed>, ErrorGuaranteed>` which could be
`flatten`ed into `Result<(), ErrorGuaranteed>`. The new result of the
`catch_fatal_errors` calls is
`Result<Result<(), ErrorGuaranteed>, FatalError>`, which can't be
`flatten`ed but is still easily matched for the success case.

By making non-unicode arguments a fatal error instead of a warning, we
don't need to handle what comes after, which avoids the need for an
`unchecked_claim_error_was_emitted` call.

`main_args` calls `from_matches`, which does lots of initialization. If
anything goes wrong, `from_matches` emits an error message and returns
`Err(1)` (or `Err(3)`). `main_args` then turns the `Err(1)` into
`Err(ErrorGuaranteed)`, because that's what `catch_with_exit_code`
requires on error. But `catch_with_exit_code` doesn't do anything with
the `ErrorGuaranteed`, it just exits with `EXIT_FAILURE`.

We can avoid the creation of the `ErrorGuaranteed` (which requires
an undesirable `unchecked_claim_error_was_emitted` call), by changing
`from_matches` to instead eagerly abort if anything goes wrong. The
behaviour from the user's point of view is the same: an early abort with
an `EXIT_FAILURE` exit code.

And we can also simplify `from_matches` to return an `Option` instead of
a `Result`:
- Old `Err(0)` case --> `None`
- Old `Err(_)` case --> fatal error.

This requires similar changes to `ScrapeExamplesOptions::new` and
`load_call_locations`.

- In `emit_producing_error_guaranteed`, only allow `Level::Error`.
- In `emit_diagnostic`, only produce `ErrorGuaranteed` for `Level` and
  `DelayedBug`. (Not `Bug` or `Fatal`. They don't need it, because the
  relevant `emit` methods abort.)
- Add/update various comments.

…teed`.

It's more to-the-point.